Bead emulsion nucleic acid amplification

ABSTRACT

Disclosed are methods for nucleic acid amplification wherein nucleic acid templates, beads, and amplification reaction solution are emulsified and the nucleic acid templates are amplified to provide clonal copies of the nucleic acid templates attached to the beads. Also disclosed are kits and apparatuses for performing the methods of the invention.

RELATED APPLICATIONS

This application claims the benefit of priority to the followingapplications: U.S. Ser. No. 60/443,471 filed Jan. 29, 2003, U.S. Ser.No. 60/465,071 filed Apr. 23, 2003; U.S. Ser. No. 60/476,313 filed onJun. 6, 2003, U.S. Ser. No. 60/476,504 filed Jun. 6, 2003, U.S. Ser. No.60/476,592 filed on Jun. 6, 2003; U.S. Ser. No. 60/476,602 filed on Jun.6, 2003, and U.S. Ser. No. 60/497,985 filed Aug. 25, 2003. All patentand patent applications in this paragraph are hereby fully incorporatedherein by reference.

This application also incorporates by reference the following copendingU.S. patent applications: “Method For Preparing Single-Stranded DNALibraries” filed Jan. 28, 2004, “Double Ended Sequencing” filed Jan. 28,2004, and “Methods Of Amplifying And Sequencing Nucleic Acids” filedJan. 28, 2004.

FIELD OF THE INVENTION

The present invention relates to methods for amplifying nucleic acidtemplates from low copy number to quantities amenable for sequencing ona solid support such as a bead. The present invention is also directedto zero bead removal—a method of enriching for solid support containingamplified nucleic acids is also disclosed.

BACKGROUND

The ability to amplify a plurality of nucleic acid sequences, such as agenomic library or a cDNA library, is critical given the inefficiency ofcurrent methods of sequencing. Current sequencing technologies requiremillions of copies of nucleic acid per sequencing reaction. Furthermore,the sequencing of a human genome would require about tens of millions ofdifferent sequencing reactions. If the starting material is limited,amplification of the initial DNA is necessary before genomic sequencing.The starting material may be limited, for example, if the genome to besequenced is from a trace quantity of pathogen or from a prenatalpatient. Current techniques for in vitro genome amplification involvelaborious cloning and culturing protocols that have limited the utilityof genomic sequencing. Other techniques, such as PCR, while fast andreliable, are unable to amplify a genome in a representative fashion.

While random primed PCR can be easily engineered to amplify a pluralityof nucleic acids in one reaction, this method is not preferred becausethe amplified library is not representative of the starting library.That is, in a random PCR environment, some DNA sequences arepreferentially amplified at the expense of other sequences such that theamplified product does not represent the starting material. This problemwith PCR may be overcome if each individual member of a library isamplified in a separate reaction. However, this approach may beimpractical if many thousands of separate reaction tubes are requiredfor the amplification process, as a genomic library or cDNA library mayinclude more than 100,000 fragments. Individual amplification of eachfragment of these libraries in separate reaction is not practical.

SUMMARY OF THE INVENTION

The present invention provides for a method of amplifying a plurality ofnucleic acids (e.g., each sequence of a DNA library, transcriptome, orgenome) in a rapid and economical manner in a single reaction tube. Oneuse of the method of the invention is to perform simultaneous clonalamplification (e.g., by PCR) of a plurality of samples (as many asseveral hundred thousand) in one reaction vessel. This invention furtherprovides means for encapsulating a plurality of DNA samples individuallyin a microcapsule of an emulsion (i.e., a microreactor), performingamplification of the plurality of encapsulated nucleic acid samplessimultaneously, and releasing said amplified plurality of DNA from themicrocapsules for subsequent reactions.

In one embodiment, single copies of the nucleic acid template speciesare hybridized to capture beads comprising, e.g., captureoligonucleotides or chemical groups that bind to the nucleic acidtemplate. The beads are suspended in complete amplification solution(see Example 2 for an example of an amplification solution) andemulsified to produce microreactors (typically 100 to 200 microns indiameter). After this, amplification (e.g., PCR) is used to clonallyincrease copy number of the initial template species in themicroreactors, and these copies bind to the capture beads in themicroreactors.

In an alternate embodiment, capture beads are added to an amplificationreaction mixture (e.g., an amplification solution from Example 2)comprising nucleic acid template and this mixture is emulsified toproduce microreactors. Amplification (e.g., PCR) is used to clonallyincrease copy number of the initial template species in themicroreactors, and these copies bind to the capture beads in themicroreactors.

One advantage of the present invention is that the microreactors allowthe simultaneous clonal and discrete amplification of many differenttemplates without cross contamination of the amplified products orreagents, or domination of one particular template or set of templates(e.g., PCR bias). The amplification reaction, for example, may beperformed simultaneously with at least 3,000 microreactors permicroliter of reaction mix. Preferably, each microreactor comprises oneor fewer species of amplified template.

In various embodiments of the invention, the microreactors have anaverage size of about 10 μm to about 250 μm. In a preferred embodiment,the microreactors have an average diameter of about 60 to about 200 μm.In a more preferred embodiment, the microreactors have an averagediameter of about 60 μm, such as an average of 40 μm to 80 μm indiameter. In an embodiment, the microreactors have an average diameterof about 60 μm. In another preferred embodiment, the microreactors havean average volume of about 113 pl. In a most preferred embodiment, about3000 microreactors are contained within a microliter of a 1:2 water tooil emulsion.

The present invention also provides for a method for producing aplurality of nucleic acid template-carrying beads wherein each beadcomprises up to and more than 1,000,000 copies of a single nucleic acidsequence. In one preferred embodiment, each bead may comprise over 20million copies of a single nucleic acid.

The present invention further provides for a library made by the methodsof the invention. The library may be made by using, e.g., a genomic DNAlibrary, a cDNA library, or a plasmid library as the starting materialfor amplification. The library may be derived from any population ofnucleic acids, e.g., biological or synthetic in origin.

The present invention also provides for a method of enriching for thosebeads that contains the product of successful DNA amplification (i.e.,by removing beads that have no DNA attached thereto).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of the structure of a DNA capture bead.

FIGS. 2A-2B Schematic of one embodiment of a bead emulsion amplificationprocess.

FIG. 3 Schematic of an enrichment process to remove beads that do nothave any DNA attached thereto.

FIG. 4 Depiction of jig used to hold tubes on the stir plate belowvertical syringe pump. The jig was modified to hold three sets of beademulsion amplification reaction mixtures. The syringe was loaded withthe PCR reaction mixture and beads.

FIG. 5 Depiction of optimal placement of syringes in vertical syringepump and orientation of emulsion tubes below syringe outlets.

FIG. 6 Depiction of optimal placement of syringe pump pusher blockagainst syringe plungers, and optimal orientation of jig on the stirplate. Using this arrangement, the syringe contents were expelled intothe agitated emulsion oil.

FIG. 7 Depiction of beads (see arrows) suspended in individualmicroreactors according to the methods of the invention.

FIGS. 8A-8C Schematic showing the initial stages of bead emulsionamplification used in conjunction with double ended sequencing. TheNHS-activated bead (FIG. 8A) is attached with capture primers (FIG. 8B),and encapsulated in a microreactor comprising the DNA capture bead andtemplate (FIG. 8C).

FIG. 9 Schematic showing the amplification and capture stages of beademulsion amplification used in conjunction with double ended sequencing.The template is amplified by solution phase PCR and the amplificationproducts are attached to the DNA capture bead.

FIG. 10 Schematic showing the later stages of bead emulsionamplification used in conjunction with double ended sequencing. Theemulsion is broken down (FIGS. 10A-10B), the second strand of theamplification product is removed and enrichment is used to maximize thenumber of beads bound with amplification product (FIG. 10C), thesequencing primers are annealed (FIG. 10D), and the first strand issequenced (FIG. 10E), followed by the second strand.

DETAILED DESCRIPTION OF INVENTION

Brief Overview Of Bead Emulsion Amplification

A brief overview of one embodiment of the invention is discussed below.A more detailed description of each individual step of this embodimentwill follow. In this embodiment, PCR is the chosen amplificationtechnique.

In one aspect of the invention, bead emulsion amplification is performedby attaching a template (e.g., DNA template) to be amplified to a solidsupport, preferably in the form of a generally spherical bead. The beadis linked to a large number of a single primer species (i.e., primer Bin FIG. 2) that is complementary to a region of the template DNA and theamplification copies of this template. Alternately, the bead is linkedto chemical groups (e.g., biotin) that can bind to chemical groups(e.g., streptavidin) included on the template DNA and amplificationcopies of this template. The beads are suspended in aqueous reactionmixture and then encapsulated in a water-in-oil emulsion. In differentaspects of the invention, the template DNA is bound to the bead prior toemulsification, or the template DNA is included in solution in theamplification reaction mixture. In a preferred embodiment, anamplification step is performed prior to distribution of the nucleicacid templates onto a multiwell (e.g., picotiter) plate.

In certain embodiments, the emulsion is composed of discrete aqueousphase microdroplets, e.g., averaging approximately 60 to 200 μm indiameter, enclosed by a thermostable oil phase. Each microdropletcontains, preferably, amplification reaction solution (i.e., thereagents necessary for nucleic acid amplification). An example of anamplification reaction solution would be a PCR reaction mixture(polymerase, salts, dNTPs; see Example 2 for an example of oneembodiment) and a pair of PCR primers (primer A and primer B). See, FIG.2A. In some cases, the template DNA is included in the reaction mixture.A subset of the microdroplet population includes the DNA bead and thetemplate. This subset of microdroplet is the basis for theamplification. The remaining microcapsules do not contain template DNAand will not participate in amplification. In one embodiment, theamplification technique is PCR and the PCR primers are present in an 8:1or 16:1 ratio (i.e., 8 or 16 of one primer to 1 of the second primer) toperform asymmetric PCR. In another embodiment, the ratio of PCR primersmay be substantially equal for normal PCR.

The amplification reaction, such as PCR, may be performedusing anysuitable method. In the following overview, one mechanism of PCR isdiscussed as an illustration. However, it is understood that theinvention is not limited to this mechanism. In the example, a region ofthe DNA molecule (B′ region) is annealed to an oligonucleotideimmobilized to a bead (primer B). During thermocycling (FIG. 2B), thebond between the single stranded template and the immobilized B primeron the bead is broken, releasing the template into the surroundingmicroencapsulated solution. The amplification solution, in this case,the PCR solution, contains addition solution phase primer A and primer B(e.g., in a 8:1 or 16:1 ratio). Solution phase B primers readily bind tothe complementary B′ region of the template as binding kinetics are morerapid for solution phase primers than for immobilized primers.

In early phase PCR, both A and B strands amplify equally well (FIG. 2C).By midphase PCR (i.e., between cycles 10 and 30) the B primers aredepleted, halting exponential amplification. The reaction then entersasymmetric amplification and the amplicon population becomes dominatedby A strands (FIG. 2D). In late phase PCR (FIG. 2E), after 30 to 40cycles, asymmetric amplification increases the concentration of Astrands in solution. Excess A strands begin to anneal to beadimmobilized B primers. Thermostable polymerases then utilize the Astrand as a template to synthesize an immobilized, bead bound B strandof the amplicon.

In final phase PCR (FIG. 2F), continued thermal cycling forcesadditional annealing to bead bound primers. Solution phase amplificationmay be minimal at this stage but concentration of immobilized B strandsincrease. Then, the emulsion is broken and the immobilized product isrendered single stranded by denaturing (by heat, pH etc.) which removesthe complimentary A strand. The A primers are annealed to the A′ regionof immobilized strand, and immobilized strand is loaded with sequencingenzymes, and any necessary accessory proteins. The beads are thensequenced using recognized pyrophosphate techniques (described, e.g., inU.S. Pat. Nos. 6,274,320, 6258,568 and 6,210,891, incorporated in totoherein by reference).

Template Design

In a preferred embodiment, the nucleic acid template to be amplified bybead emulsion amplification is a population of DNA such as, for example,a genomic DNA library or a cDNA library. It is preferred that eachmember of the DNA population have a common nucleic acid sequence at thefirst end and a common nucleic acid sequence at a second end. This canbe accomplished, for example, by ligating a first adaptor DNA sequenceto one end and a second adaptor DNA sequence to a second end of eachmember of the DNA population. Many DNA and cDNA libraries, by nature ofthe cloning vector (e.g., Bluescript, Stratagene, La Jolla, Calif.) fitthis description of having a common sequence at a first end and a secondcommon sequence at a second end of each member DNA. The nucleic acidtemplate may be of any size amenable to in vitro amplification(including the preferred amplification techniques of PCR and asymmetricPCR). In a preferred embodiment, the template is about 150 to 750 bp insize, such as, for example about 250 bp in size.

Binding Nucleic Acid Template to Capture Beads

In one aspect of the invention, a single stranded nucleic acid templateto be amplified is attached to a capture bead. The template may becaptured to the bead prior to emulsification or after the emulsion hasbeen formed. In a preferred aspect, the amplification copies of thenucleic acid template are attached to a capture bead. As non-limitingexamples, these attachments may be mediated by chemical groups oroligonucleotides that are bound to the surface of the bead. The nucleicacid (e.g., the nucleic acid template, amplification copies, oroligonucleotides) may be attached to the solid support (e.g., a capturebead) in any manner known in the art.

According to the present invention, covalent chemical attachment of anucleic acid to the bead can be accomplished by using standard couplingagents. For example, water-soluble carbodiimide can be used to link the5′-phosphate of a DNA sequence to amine-coated capture beads through aphosphoamidate bond. Alternatively, specific oligonucleotides can becoupled to the bead using similar chemistry, and to then DNA ligase canbe used to ligate the DNA template to the oligonucleotide on the bead.Other linkage chemistries to join the oligonucleotide to the beadsinclude the use of N-hydroxysuccinamide (NHS) and its derivatives.

In an exemplary method, one end of a linker may contain a reactive group(such as an amide group) which forms a covalent bond with the solidsupport, while the other end of the linker contains a second reactivegroup that can bond with the oligonucleotide to be immobilized. In apreferred embodiment, the oligonucleotide is bound to the DNA capturebead by covalent linkage. However, non-covalent linkages, such aschelation or antigen-antibody complexes, may also be used to join theoligonucleotide to the bead.

As non-limiting examples, oligonucleotides can be employed whichspecifically hybridize to unique sequences at the end of the DNAfragment, such as the overlapping end from a restriction enzyme site orthe “sticky ends” of cloning vectors, but blunt-end linkers can also beused. These methods are described in detail in U.S. Pat. No. 5,674,743.It is preferred that the beads will continue to bind the immobilizedoligonucleotide throughout the steps in the methods of the invention.

In one embodiment of the invention, each capture bead is designed tohave a plurality of oligonucleotides that recognize (i.e., arecomplementary to) a portion of the nucleic template, and theamplification copies of this template. In the methods described herein,clonal amplification of the template species is desired, so it ispreferred that only one unique nucleic acid species is attached to anyone capture bead.

The beads used herein may be of any convenient size and fabricated fromany number of known materials. Example of such materials include:inorganics, natural polymers, and synthetic polymers. Specific examplesof these materials include: cellulose, cellulose derivatives, acrylicresins, glass, silica gels, polystyrene, gelatin, polyvinyl pyrrolidone,co-polymers of vinyl and acrylamide, polystyrene cross-linked withdivinylbenzene or the like (as described, e.g., in Merrifield,Biochemistry 1964, 3, 1385-1390), polyacrylamides, latex gels,polystyrene, dextran, rubber, silicon, plastics, nitrocellulose, naturalsponges, silica gels, control pore glass, metals, cross-linked dextrans(e.g., Sephadex™) agarose gel (Sepharose™), and other solid phasesupports known to those of skill in the art. In preferred embodiments,the capture beads are beads approximately 2 to 100 μm in diameter, or 10to 80 μm in diameter, most preferably 20 to 40 μm in diameter. In apreferred embodiment, the capture beads are Sepharose beads.

Emulsification

For use with the present invention, capture beads with or withoutattached nucleic acid template are suspended in a heat stablewater-in-oil emulsion. It is contemplated that a plurality of themicroreactors include only one template and one bead. There may be manydroplets that do not contain a template or which do not contain a bead.Likewise there may be droplets that contain more than one copy of atemplate. The emulsion may be formed according to any suitable methodknown in the art. One method of creating emulsion is described below butany method for making an emulsion may be used. These methods are knownin the art and include adjuvant methods, counter-flow methods,cross-current methods, rotating drum methods, and membrane methods.Furthermore, the size of the microcapsules may be adjusted by varyingthe flow rate and speed of the components. For example, in dropwiseaddition, the size of the drops and the total time of delivery may bevaried. Preferably, the emulsion contains a density of about 3,000 beadsencapsulated per microliter.

Various emulsions that are suitable for biologic reactions are referredto in Griffiths and Tawfik, EMBO, 22, pp. 24-35 (2003); Ghadessy et al.,Proc. Natl. Acad. Sci. USA 98, pp. 4552-4557 (2001); U.S. Pat. No.6,489,103 and WO 02/22869, each fully incorporated herein by reference.It is noted that Griffiths et al., (U.S. Pat. No. 6,489,103 and WO99/02671) refers to a method for in vitro sorting of one or more geneticelements encoding a gene products having a desired activity. This methodinvolves compartmentalizing a gene, expressing the gene, and sorting thecompartmentalized gene based on the expressed product. In contrast tothe present invention, the microencapsulated sorting method of Griffithis not suitable for parallel analysis of multiple microcapsules becausetheir nucleic acid product is not anchored and cannot be anchored. Sincethe nucleic acids of Griffiths are not anchored, they would be mixedtogether during demulsification.

The emulsion is preferably generated by adding beads to an amplificationsolution. As used herein, the term “amplification solution” means thesufficient mixture of reagents that is necessary to performamplification of template DNA. One example of an amplification solution,a PCR amplification solution, is provided in the Examples below. It willbe appreciated that various modifications may be made to theamplification solution based on the type of amplification beingperformed and whether the template DNA is attached to the beads orprovided in solution. In one embodiment, the mixture of beads andamplification solution is added dropwise into a spinning mixture ofbiocompatible oil (e.g., light mineral oil, Sigma) and allowed toemulsify. In another embodiment, the beads and amplification solutionare added dropwise into a cross-flow of biocompatible oil. The oil usedmay be supplemented with one or more biocompatible emulsion stabilizers.These emulsion stabilizers may include Atlox 4912, Span 80, and otherrecognized and commercially available suitable stabilizers. In preferredaspects, the emulsion is heat stable to allow thermal cycling, e.g., toat least 94° C., at least 95° C., or at least 96° C. Preferably, thedroplets formed range in size from about 5 microns to about 500 microns,more preferably from about 10 microns to about 350 microns, even morepreferably from about 50 to 250 microns, and most preferably from about100 microns to about 200 microns. Advantageously, cross-flow fluidmixing allows for control of the droplet formation, and uniformity ofdroplet size. We note that smaller water droplets not containing beadsmay be present in the emulsion.

The microreactors should be sufficiently large to encompass sufficientamplification reagents for the degree of amplification required.However, the microreactors should be sufficiently small so that apopulation of microreactors, each containing a member of a DNA library,can be amplified by conventional laboratory equipment, e.g., PCRthermocycling equipment, test tubes, incubators and the like. Notably,the use of microreactors allows amplification of complex mixtures oftemplates (e.g., genomic DNA samples or whole cell RNA) withoutintermixing of sequences, or domination by one or more templates (e.g.,PCR selection bias; see, Wagner et al., 1994, Suzuki and Giovannoni,1996; Chandler et al., 1997, Polz and Cavanaugh, 1998).

With the limitations described above, the optimal size of a microreactormay be on average 100 to 200 microns in diameter. Microreactors of thissize would allow amplification of a DNA library comprising about 600,000members in a suspension of microreactors of less than 10 ml in volume.For example, if PCR is the chosen amplification method, 10 ml ofmicroreactors would fit into 96 tubes of a regular thermocycler with 96tube capacity. In a preferred embodiment, the suspension of 600,000microreactors would have a volume of less than 1 ml. A suspension ofless than 1 ml may be amplified in about 10 tubes of a conventional PCRthermocycler. In a most preferred embodiment, the suspension of 600,000microreactors would have a volume of less than 0.5 ml.

Another embodiment of the invention is directed to a method ofperforming nucleic acid amplification with a template and a bead, butwithout attachment of the template to the bead. In one aspect, the beadmay comprise a linker molecule that can bind the amplified nucleic acidafter amplification. For example, the linker may be a linker that can beactivated. Such linkers are well known and include temperature sensitiveor salt sensitive binding pairs such as streptavidin/biotin andantibodies/antigen. The template nucleic acid may be encapsulated with abead and amplified. Following amplification, the amplified nucleic acidmay be linked to the beads, e.g., by adjustments in temperature or saltconcentration.

Amplification

After encapsulation, the template nucleic acid may be amplified, whileattached or unattached to beads, by any suitable method of amplificationincluding transcription-based amplification systems (Kwoh D. et al.,Proc. Natl. Acad Sci. (U.S.A.) 86:1173 (1989); Gingeras T. R. et al., WO88/10315; Davey, C. et al., EP Publication No. 329,822; Miller, H. I. etal., WO 89/06700), “RACE” (Frohman, M. A., In: PCR Protocols: A Guide toMethods and Applications, Academic Press, NY (1990)) and one-sided PCR(Ohara, O. et al., Proc. Natl. Acad. Sci. (U.S.A.) 86.5673-5677 (1989)).Still other methods such as di-oligonucleotide amplification, isothermalamplification (Walker, G. T. et al., Proc. Natl. Acad. Sci. (U.S.A.)89:392-396 (1992)), Nucleic Acid Sequence Based Amplification (NASBA;see, e.g., Deiman B et al., 2002, Mol Biotechnol. 20(2):163-79),whole-genome amplification (see, e.g., Hawkins T L et al., 2002, CurrOpin Biotechnol. 13(1):65-7), strand-displacement amplification (see,e.g., Andras S C, 2001, Mol Biotechnol. 19(1):29-44), rolling circleamplification (reviewed in U.S. Pat. No. 5,714,320), and other wellknown techniques may be used in accordance with the present invention.

In a preferred embodiment, DNA amplification is performed by PCR. PCRaccording to the present invention may be performed by encapsulating thetarget nucleic acidwith a PCR solution comprising all the necessaryreagents for PCR. Then, PCR may be accomplished by exposing the emulsionto any suitable thermocycling regimen known in the art. In a preferredembodiment, 30 to 50 cycles, preferably about 40 cycles, ofamplification are performed. It is desirable, but not necessary, thatfollowing the amplification procedure there be one or more hybridizationand extension cycles following the cycles of amplification. In apreferred embodiment, 10 to 30 cycles, preferably about 25 cycles, ofhybridization and extension are performed (e.g., as described in theexamples). Routinely, the template DNA is amplified until typically atleast 10,000 to 50,000,000 copies are immobilized on each bead. It isrecognized that for nucleic acid detection applications, fewer copies oftemplate are required. For nucleic acid sequencing applications weprefer that at least two million to fifty million copies, preferablyabout ten million to thirty million copies of the template DNA areimmobilized on each bead. The skilled artisan will recognize that thesize of bead (and capture site thereon) determines how many captiveprimers can be bound (and thus how many amplified templates may becaptured onto each bead).

PCR Primer Design

The selection of nucleic acid primers for amplification, such as PCRamplification, is well within the abilities of one of skill in the art.Strategies for primer design may be found throughout the scientificliterature, for example, in Rubin, E. and A. A. Levy, Nucleic Acids Res,1996. 24(18): p. 3538-45; and Buck, G. A., et al., Biotechniques, 1999.27(3): p. 528-36. In a preferred embodiment, primers can be limited to alength of 20 bases (5 tetramers) for efficient synthesis of bipartitePCR/sequencing primers. Each primer can include a two-base GC clamp onthe 5′ end, a single GC clamp on the 3′ end, and all primers can sharesimilar T_(m) (±2° C.). In a preferred embodiment, hairpin structureswithin the primers (internal hairpin stems ΔG>−1.9 kcal/mol) arestrongly discouraged in any of the designed primers. In anotherpreferred embodiment, primer dimerization is also controlled; such thata 3-base maximum acceptable dimer is allowed. However, this is allowedto occur only in the final six 3′ bases, and the maximum allowable ΔGfor a 3′ dimer is −2.0 kcal/mol. Preferably, a penalty is applied toprimers in which the 3′ ends are too similar to others in the group.This prevents cross-hybridization between one primer and the reversecomplement of another primer.

If the primers are designed according to the criteria described above,the possibility of complimentary regions occurring within the genome ofinterest is not of major concern, despite the reported tolerance of PCRto mismatches in complex sample populations (Rubin, E. and A. A. Levy.Nucleic Acids Res, 1996. 24(18): p. 3538-45). Although the probabilityof finding a perfect match to a 20 base primer is extremely low (4²⁰)(see Table 1), the probability of finding shorter non-consecutivematches increases significantly with the size of the genome of interest.As a result, the probability of finding a perfect match for a sequenceof at least 10 of 20 bases is 99.35% for an Adenovirus genome. Theprobability of finding a perfect match for a sequence of 16 bases is 97%for the sequences in the NCBI database (approximately 100 times moresequence information than the Adenovirus genome). The probability offinding a perfect match for a sequence of 17 to 20 bases is 99% for thehuman genome (approximately 3 billion bases). TABLE 1 The probability ofperfect sequence matches for primers increases with decreasing matchlength requirements and increasing size of the genome of interest.Perfect match % chance for match % chance for match % chance for matchMatch probability in Adeno ˜35 K in NCBI bacterial in Human ˜3 B Length(1/(4{circumflex over ( )}length)) bases database ˜488 M bases bases 209.1E−13  0.00%  0.04%  0.27% 19 7.3E−12  0.00%  0.65%  4.32% 18 4.4E−11 0.00%  5.76%  34.37%  17 2.3E−10  0.00%  35.69%   99.17%  16 1.2E−09 0.02%  97.52%  >100% 15 5.6E−09  0.12% >100% >100% 14 2.6E−08 0.64% >100% >100% 13 1.2E−07  3.29% >100% >100% 12 5.4E−07 15.68%  >100% >100% 11 2.4E−06  58.16%  >100% >100% 10 1.0E−05 99.35%  >100% >100% 9 4.6E−05  99.77%  >100% >100% 82.0E−04 >100% >100% >100% 7 8.5E−04 >100% >100% >100% 63.7E−03 >100% >100% >100% 5 1.6E−02 >100% >100% >100% 46.4E−02 >100% >100% >100% 3 2.5E−01 >100% >100% >100% 27.1E−01 >100% >100% >100% 1 1.0E+00 >100% >100% >100%

However, primer cross-hybridization to various regions of the genome isless problematic than one might expect due to the random DNA digestionused to form the nucleic acid templates. The cross-hybridizing regions(CHRs) are fairly benign. First, it is unlikely that a CHR would be ableto successfully compete with the perfect match between the PCR primersin solution and the template. In addition, any primers that includemismatches at their 3′ end will be at a significant competitivedisadvantage. Even if a CHR should out compete the intended PCR primer,it would produce a truncated PCR product, without a downstream site forthe sequencing primer. If the truncated product could be driven to thecapture bead and immobilized, one of two situations would result. If theCHR out-competed the solution-phase primer, then the immobilized productwould lack a sequencing primer binding site, and would result in anempty picotiter plate (PTP) well. If the CHR out-competed the bead-boundprimer, the sequencing primer would still be present, and the onlyeffect would be a shorter insert. Neither result would unduly compromisethe sequencing quality. Given the large amount of genomic material usedin the sample preparation process (currently 25 μg, containing 5.29×10¹⁶copies of the 35 Kb Adenovirus genome), oversampling can be used toprovide fragments that lack the complete CHR, and allow standard PCRamplification of the region in question.

Breaking the Emulsion and Bead Recovery

Following amplification of the nucleic acid template and the attachmentof amplification copies to the bead, the emulsion is “broken” (alsoreferred to as “demulsification” in the art). There are many methods ofbreaking an emulsion (see, e.g., U.S. Pat. No. 5,989,892 and referencescited therein) and one of skill in the art would be able to select anappropriate method. In the present invention, one preferred method ofbreaking the emulsion uses additional oil to cause the emulsion toseparate into two phases. The oil phase is then removed, and a suitableorganic solvent (e.g., hexanes) is added. After mixing, the oil/organicsolvent phase is removed. This step may be repeated several times.Finally, the aqueous layers above the beads are removed. The beads arethen washed with a mixture of an organic solvent and annealing buffer(e.g., one suitable annealing buffer is described in the examples), andthen washed again in annealing buffer. Suitable organic solvents includealcohols such as methanol, ethanol, and the like.

The beads bound to amplification products may then be resuspended inaqueous solution for use, for example, in a sequencing reactionaccording to known technologies. (See, Sanger, F. et al., Proc. Natl.Acad. Sci. U.S.A. 75, 5463-5467 (1977); Maxam, A. M. & Gilbert, W. ProcNatl Acad Sci USA 74, 560-564 (1977); Ronaghi, M. et al., Science 281,363, 365 (1998); Lysov, I. et al., Dokl Akad Nauk SSSR 303, 1508-1511(1988); Bains W. & Smith G. C. J. Theor Biol 135, 303-307(1988); Dmanac,R. et al., Genomics 4, 114-128 (1989); Khrapko, K. R. et al., FEBS Lett256. 118-122 (1989); Pevzner P. A. J Biomol Struct Dyn 7, 63-73 (1989);Southern, E. M. et al., Genomics 13, 1008-1017 (1992).)

If the beads are to be used in a pyrophosphate-based sequencing reaction(described, e.g., in U.S. Pat. Nos. 6,274,320, 6258,568 and 6,210,891,and incorporated in toto herein by reference), then it is necessary toremove the second strand of the PCR product and anneal a sequencingprimer to the single stranded template that is bound to the bead. Thesecond strand may be melted away using any number of commonly knownmethods such as addition of NaOH, application of low ionic (e.g., salt)strength, enzymatic degradation or displacement of the second strand, orheat processing. Following this strand removal step, the beads arepelleted and the supernatant is discarded. The beads are resuspended inan annealing buffer, and a sequencing primer or other non-amplificationprimer is added. The primer is annealed to the single strandedamplification product. This can be accomplished by using an appropriateannealing buffer and temperature conditions, e.g., as according tostandard procedures in the art.

Purifying the Beads

At this point, the amplified nucleic acid on the bead may be sequencedeither directly on the bead or in a different reaction vessel. In anembodiment of the present invention, the nucleic acid is sequenceddirectly on the bead by transferring the bead to a reaction vessel andsubjecting the nucleic acid to a sequencing reaction (e.g.,pyrophosphate or Sanger sequencing). Alternatively, the beads may beisolated and the nucleic acid may be removed from each bead andsequenced. In either case, the sequencing steps may be performed on eachindividual bead. However, this method, while conunercially viable andtechnically feasible, may not be most effective because many of thebeads will be “negative” beads (i.e., beads without amplified nucleicacid attached). In such cases, the optional process outlined below maybe used to remove negative beads prior to distribution onto multiwell(e.g., picotiter) plates.

A high percentage of the beads may be negative if the goal is tominimize the number of beads that are associated with two or moredifferent species of nucleic acid templates. For optimal pyrophosphatesequencing, each bead should contain multiple copies of a single speciesof nucleic acid. This can be achieved by maximizing the total number ofbeads combined with a single fragment of nucleic acid beforeamplification. For example, the following mathematical model can beused.

For the general case of N number of DNAs randomly distributed with Mnumber of beads, the relative bead population associated with any numberof DNAs depends on the ratio of N/M. The fraction of beads associatedwith N DNAs R(N) may be calculated using the Poisson distribution:R(N)=exp−(N/M)×(N/M)^(N) /N! (where x is the multiplication symbol)

Table 2, below, shows some calculated values for various N/M (theaverage DNA fragment-to-bead ratio) and N (the number of fragmentsassociated with a bead). TABLE 2 N/M 0.1 0.5 1 2 R(0) 0.9 0.61 0.37 0.13R(1) 0.09 0.3 0.37 0.27 R(N > 1) 0.005 0.09 0.26 0.59

In Table 2, the top row denotes the various ratios of N/M. R(0) denotesthe fraction of beads with no DNA, R(1) denotes the fraction of beadswith one DNA (before amplification), and R(N>1) denotes the fraction ofDNA with more than one DNA (before amplification).

Table 2 indicates that the maximum fraction of beads associated with asingle DNA fragment is 0.37 (37%) and this occurs at a fragment-to-beadratio of one-to-one. In this mixture, about 63% of the beads cannot beused for sequencing because they are associated with no DNA or they areassociated with more than one species of DNA. However, controlling thefragment-to-bead ratio requires complex calculations, and variabilitycan produce bead batches with a significantly smaller fraction ofuseable beads.

This inefficiency can be significantly ameliorated if beads containingamplicon (originating from the association with at least one fragment)are separated from those without amplicon (originating from beads withno associated fragments). An amplicon is defined as any nucleic acidmolecules produced by an in vitro nucleic amplification technique. Toincrease efficiency, binding can be performed usinglow fragment-to-beadratios (N/M<1). This minimizes the number of beads associated with morethan one DNA. A separation step can be used to remove most or all of thebeads with no DNA, leaving an enriched population of beads with one ormore species of amplified DNA. This enriched population may be analyzedby any method of sequencing such as, for example, pyrophosphatesequencing. Because the fraction of beads with one amplicon (N=1) isenriched, any method of sequencing can be applied more efficiently.

As an example, with an average fragment-to-bead ratio of 0.1, 90% of thebeads will carry no amplicon, 9% of the beads will carry one amplicon,and 0.5% of the beads will carry more than one amplicon. The enrichmentdescribed herein below will remove the 90% of the zero amplicon beadsleaving a population of beads where the fraction available forsequencing (N=1) is:1−(0.005/0.09)=94%.

Dilution of the fragment to bead mixture, along with separation of beadscontaining amplicon can yield an enrichment of 2.5 fold over the optimalunenriched method. For example, 94%/37% (See Table 2, above, N/M=1)=2.5.An additional benefit of the enrichment procedure described herein belowis that the ultimate fraction of beads useful for sequencing isrelatively insensitive to variability in N/M. Thus, complex calculationsto derive the optimal N/M ratio are either unnecessary or may beperformed with lower levels of precision. Accordingly, the methods ofthe invention can be easily adapted for use by less trained personnel orautomation. An additional benefit of these methods is that the zeroamplicon beads may be recycled and reused. While recycling is notnecessary, it may reduce cost or the total bulk of reagents making themethod of the invention more suitable for some purposes such as, forexample, portable sampling, remote robotic sampling, and the like. Inaddition, the collective benefits of the disclosed methods (e.g.,adaptation for less trained personnel, automation, and recycling ofreagents) will reduce the costs of the methods. The enrichment procedureis described in more detail below.

The enrichment procedure may be used to treat beads that have beenamplified in the bead emulsion method described above. The amplificationis designed so that each amplified nucleic acid molecule contains thesame sequence at its 3′ end. The nucleotide sequence may be a 20 mer butmay be any sequence from 15 bases or more such as 25 bases, 30 bases, 35bases, 40 bases, or longer. While longer oligonucleotide ends arefunctional, they are not necessary. This 3′ sequence may be introducedat the end of an amplified nucleic acid by one of skill in the art. Forexample, if PCR is used for amplification of a DNA template, thesequence may be included as part of one member of the PCR primer pair.

A schematic of the enrichment process is depicted in FIG. 3. In thisprocess, the amplicon-bound bead is mixed with four empty beads tocreate a fragment-diluted amplification bead mixture. In step 1, abiotinylated primer complementary to the 3′ end of the amplicon isannealed to the amplicon. In step 2, DNA polymerase and the four naturaldeoxynucleotide triphosphates (dNTPs) are added to the bead mixture andthe biotinylated primer is extended. This extension is to enhance thebonding between the biotinylated primer and the bead-bound DNA. Thisstep may be omitted if the biotinylated primer—DNA bond is strong (e.g.,in a high ionic environment). In Step 3, streptavidin coated beadssusceptible to attraction by a magnetic field (referred to herein as“magnetic streptavidin beads”) are introduced to the bead mixtures.Magnetic beads are commercially available, for example, from Dynal(M290). The streptavidin capture moieties binds biotin groups hybridizedto the amplicons, thereby binding the amplicon-bound beads to themagnetic streptavidin beads.

In step 5, a magnetic field (represented by a magnet) is applied nearthe reaction mixture, which causes the magnetic streptavidinbeads/amplicon bound bead complexes to be positioned along one side ofthe tube most proximal to the magnetic field. Magnetic beads withoutamplicon bound beads attached are also expected to be positioned alongthe same side. Beads without amplicons remain in solution. The beadmixture is washed and the beads not bound by the magnet (i.e., the emptybeads) are removed and discarded. In step 6, the extended biotinylatedprimer strand is separated from the amplicon strand by “melting.” Thisstep that can be accomplished, for example, by heat or a change in pH.The heat may be 60° C. in low salt conditions (e.g., in a low ionicenvironment such as 0.1×SSC). The change in pH may be accomplished bythe addition of NaOH. Next, the mixture is washed and the supernatantcontaining the amplicon bound beads is recovered, while the magneticbeads are retained by a magnetic field. The resultant enriched beads maybe used for DNA sequencing. It is noted that the primer on the DNAcapture bead may be the same as the primer of step 2, above. In thiscase, annealing of the amplicon-primer complementary strands (with orwithout extension) is the source of target-capture affinity.

The biotin streptavidin pair could be replaced by a variety ofcapture-target pairs. For example, capture-target pairs can employreversible (e.g., cleavable) or irreversible linkages. Non-limitingexamples of reversible linkages include thiol-thiol,digoxigenin/anti-digoxigenin, and linkages using VECTREX® Avidin DLA(Vector Laboratories, Burlingame, Calif.), CaptAvidin™, NeutrAvidin™,and D-desthiobiotin (Molecular Probes, Inc., Eugene, Oreg.).

As described above, step 2 of the enrichment process is optional. Ifstep 2 is omitted, it may not be necessary to separate the magneticbeads from the amplicon bound beads. The amplicon bound beads, with themagnetic beads attached, may be used directly for sequencing. Forexample, separation may not be necessary if sequencing is to beperformed in a microtiter or picotiter plate and the amplicon boundbead-magnetic bead complex can fit inside the well of the plate.

While the use of magnetic capture beads is convenient, capture moietiescan encompass other binding surfaces. For example, streptavidin can bechemically bound to a surface such as the inner surface of a tube. Inthis case, the amplified bead mixture may be flowed through the tube.The amplicon bound beads will tend to be retained until “melting” whilethe empty beads will flow through. This arrangement may be particularlyadvantageous for automating the bead preparation process.

While the embodiments described above are particularly useful, othermethods to separate beads can be envisioned. For example, the capturebeads may be labeled with a fluorescent moiety which would make thetarget-capture bead complex fluorescent. The target capture bead complexmay be separated by flow cytometry or fluorescence cell sorter. Usinglarge capture beads would allow separation by filtering or otherparticle size separation techniques. Since both capture and target beadsare capable of forming complexes with a number of other beads, it ispossible to agglutinate a mass of cross-linked capture-target beads. Thelarge size of the agglutinated mass would make separation possible bysimply washing away the unagglutinated empty beads. These methodsdescribed are described in more detail, for example, in Bauer, J.; J.Chromatography B, 722 (1999) 55-69 and in Brody et al., Applied PhysicsLett. 74 (1999) 144-146.

In one embodiment, the invention encompasses a method for amplifying oneor more nucleic acids comprising the steps of: a) forming a water-in-oilemulsion to create a plurality of aqueous microreactors wherein at leastone of the microreactors comprises a single nucleic acid template, asingle bead capable of binding to the nucleic acid, and amplificationreaction solution containing reagents necessary to perform nucleic acidamplification; b) amplifying the nucleic acids in the microreactors toform amplified copies of the nucleic acids; and c) binding the amplifiedcopies to the beads in the microreactors.

The amplification reaction solution used with this method may be apolymerase chain reaction solution comprising nucleotide triphosphates,a thermostable polymerase, and nucleic acid primers suspended in abuffer compatible with polymerase chain reaction conditions. Thepolymerase chain reaction is may be an asymmetric polymerase chainreaction or a symmetric polymerase chain reaction. As examples,amplification may be carried out by transcription-based amplification,rapid amplification of cDNA ends, continuous flow amplification, orrolling circle amplification.

For use with this method, a majority of the microreactors may include asingle nucleic acid. The method may be performed with at least 10,000nucleic acids, or at least 50,000 nucleic acids. Each bead used with themethod can be used to capture more than 10,000 amplification copies of anucleic acid template. In various embodiments, the emulsion additionallycontains emulsion stabilizers. The emulsion stabilizers may be Atlox4912, Span 80, or combinations or mixtures thereof. The emulsion may beheat stable, e.g., to 95° C., and may be formed by the dropwise additionof the nucleic acid templates, beads, and amplification reactionsolution into an oil. The microreactors may have an average size of 50to 250 μm in diameter.

In another embodiment, the invention encompasses a library comprising aplurality of nucleic acid molecules, wherein each nucleic acid moleculeis separately immobilized to a different bead, and wherein each beadcomprises over 1,000,000 clonal amplification copies of each nucleicacid molecule, wherein the library is contained in a single vessel. Asexamples, the nucleic acid molecules may be genomic DNA, cDNA, episomalDNA, BAC DNA, or YAC DNA. The genomic DNA may be animal, plant, viral,bacterial, or fungal genomic DNA. Preferably, the genomic DNA is humangenomic DNA or human cDNA. In certain aspects, the bead, e.g., aSepharose bead, has a diameter of 2 microns to 100 microns.

The invention also encompasses a method for amplifying a nucleic acidcomprising the steps of: a) providing a nucleic acid template to beamplified; b) providing a solid support material comprising a generallyspherical bead having a diameter about 10 to about 80 m, wherein thebead is capable of binding to the nucleic acid template; c) mixing thenucleic acid template and the bead in an amplification reaction solutioncontaining reagents necessary to perform a nucleic acid amplificationreaction in a water-in-oil emulsion; d) amplifying the nucleic acidtemplate to form amplified copies of the nucleic acid template; and e)binding the amplified copies to the bead.

As an option, the method can include an enrichment step to isolate beadswhich bind amplified copies of the nucleic acid away from beads to whichno nucleic acid is bound. This enrichment step may be performed byelectrophoresis, cell sorting, or affinity purification (e.g., withmagnetic beads that bind nucleic acid). Preferably, at least 100,000copies of each target nucleic acid molecule are bound to each bead, atleast 1,000,000 copies of each target nucleic acid molecule are bound toeach bead, or at least 1 to 20,000,000 copies of each target nucleicacid molecule are bound to each bead. In various aspects, the beads areSepharose beads and amplified copies are bound to the beads by a bindingpair such as antigen/antibody, ligand/receptor, polyhistidine/nickel, oravidin/biotin. The method can also include the steps of: f) separatingthe template carrying beads and magnetic bead; and g) removing themagnetic beads with a magnetic field. This separation may be achieved byincubation at a temperature greater than 45° C. or by incubating thetemplate carrying beads' and the magnetic beads in a solution with abasic pH.

The invention further encompasses a kit for conducting nucleic acidamplification of a nucleic acid template comprising: a) a nucleic acidcapture bead; b) an emulsion oil; c) one or more emulsion stabilizers;and d) instructions for employing the kit.

Additionally, the invention encompasses a method for producing a clonalpopulation of nucleic acids, comprising: a) providing a plurality ofnucleic acid templates from 50-800 bp in length and beads capable ofbinding to the nucleic acid templates; b) mixing the nucleic acidtemplates and the beads in a biological reaction solution containingreagents necessary to amplify the nucleic acid templates; and c) formingan emulsion to create a plurality of microreactors comprising thenucleic acid templates, beads, and biological reaction solution, whereinat least one of the microreactors comprises a single nucleic acidtemplate and a single bead encapsulated in the biological reactionsolution, wherein the microreactors are contained in the same vessel.

In accordance with this method, the nucleic acids can be transcribed andtranslated to generate at least 10,000 copies of an expression product.The expression product may be bound to the beads by a binding pairselected from the group consisting of antigen/antibody, ligand/receptor,6×his/nickel-nitrilotriacetic acid, and FLAG tag/FLAG antibody bindingpairs. In certain aspects, the method produces a clonal population ofproteins, such as antibodies, antibodies fragments, and engineeredantibodies. The emulsion may comprise a plurality of thermostablemicroreactors, wherein the microreactors are 50 to 200 μm in diameterand comprise a biological reaction solution. The biological reactionsolution may comprise reagents for performing polymerase chain reactionamplification reactions or coupled transcription and translationreactions. Preferably, a plurality of microreactors comprise a nucleicacid template, e.g., one or fewer nucleic acid templates, and one orfewer beads that bind to the nucleic acid templates.

EXAMPLES

Bead Emulsion PCR

The following procedures, including capture of the template DNA, DNAamplification, and recovery of the beads bound to amplified template,can be performed in a single tube. The emulsion format ensures thephysical separation of the beads into 100-200 μm “microreactors” withinthis single tube, thus allowing for clonal amplification of the varioustemplates. Mobilization of the amplification product is achieved throughextension of the template along the oligonucleotides bound to the DNAcapture beads. Typical, the copy number of the immobilized templateranges from 10 to 30 million copies per bead. The DNA capture beadsaffixed with multiple copies of a single species of nucleic acidtemplate are ready for distribution onto PTPs.

The 300,000 75-picoliter wells etched in the PTP surface provide aunique array for the sequencing of short DNA templates in a massivelyparallel, efficient and cost-effective manner. However, this requiresfairly large quantities (millions of copies) of clonal templates in eachreaction well. The methods of the invention allow the user to clonallyamplify single-stranded genomic template species thorough PCR reactionsconducted in standard tubes or microtiter plates. Single copies of thetemplate species may be mixed with capture beads, resuspended intocomplete PCR amplification solution, and emulsified into microreactors(100 to 200 μm in diameter), after which PCR amplification generates10⁷-fold amplification of the initial template species. This procedureis much simpler and more cost-effective than previous methods.

Example 1 Binding Nucleic Acid Template to Capture Beads

This example describes preparation of a population of beads thatpreferably have only one unique nucleic acid template attached thereto.Successful clonal amplification depends on the delivery of a controllednumber of template species (0.5 to 1) to each bead. Delivery of excessspecies can result in PCR amplification of a mixed template population,preventing generation of meaningful sequence data while a deficiency ofspecies will result in fewer wells containing template for sequencing.This can reduce the extent of genome coverage provided by the sequencingphase. As a result, it is preferred that the template concentration beaccurately determined through replicated quantitation, and that thebinding protocol be followed as outlined below.

Template Quality Control

The success of the Emulsion PCR reaction is related to the quality ofthe template species. Regardless of the care and detail paid to theamplification phase, poor quality templates will impede successfulamplification and the generation of meaningful sequence data. To preventunnecessary loss of time and money, it is important to check the qualityof the template material before initiating the Emulsion PCR phase of theprocess. Preferably, the library should pass two quality control stepsbefore it is used in Emulsion PCR. Its concentration and thedistribution of products it contains should be determined. Ideally, thelibrary should appear as a heterogeneous population of fragments withlittle or no visible adapter dimers (e.g., ˜90 bases). Also,amplification with PCR primers should result in a product smear ranging,for example, from 300 to 500 bp. Absence of amplification product mayreflect failure to properly ligate the adaptors to the template, whilethe presence of a single band of any size may reflect contamination ofthe template.

Preparation of the PCR solution

The main consideration for this phase is to prevent contamination of thePCR reaction mixture with stray amplicons. Contamination of the PCRreactions with a residual amplicon is one of the critical issues thatcan cause failure of a sequencing run. To reduce the possibility ofcontamination, proper lab technique should be followed, and reactionmixture preparation should be conducted in a clean room in a UV-treatedlaminar flow hood.

PCR Reaction Mix:

For 200 μl PCR reaction mixture (enough for amplifying 600,000 beads),the following reagents were combined in a 0.2 ml PCR tube: TABLE 3 StockFinal Microliters HIFI Buffer  10 X 1 X 20 treated nucleotides  10 mM   1 mM 20 Mg  50 mM    2 mM 8 BSA  10%  0.1% 2 Tween 80  1%  0.01% 2Ppase  2 U 0.003 U 0.333333 Primer MMP1a 100 μM 0.625 μM 1.25 PrimerMMP1b  10 μM 0.078 μM 1.56 Taq polymerase  5 U  0.2 U 8 Water 136.6Total 200

The tube was vortexed thoroughly and stored on ice until the beads areannealed with template.

DNA Capture Beads:

1. 600,000 DNA capture beads were transferred from the stock tube to a1.5 ml microfuge tube. The exact amount used will depend on beadconcentration of formalized reagent.

2. The beads were pelleted in a benchtop mini centrifuge and supernatantwas removed.

3. Steps 4-11 were performed in a PCR Clean Room.

4. The beads were washed with 1 mL of 1× Annealing Buffer.

5. The capture beads were pelleted in the microcentrifuge. The tube wasturned 180° and spun again.

6. All but approximately 10 μl of the supernatant was removed from thetube containing the beads. The beads were not disturbed.

7. 1 mL of 1× Annealing Buffer was added and this mixture was incubatedfor 1 minute. The beads were then pelleted as in step 5.

8. All but approximately 100 μL of the material from the tube wasremoved.

9. The remaining beads and solution were transferred to a PCR tube.

10. The 1.5 mL tube was washed with 150 μL of 1× Annealing Buffer bypipetting up and down several times. This was added to the PCR tubecontaining the beads.

11. The beads were pelleted as in step 5 and all but 10 μL ofsupernatant was removed, taking care to not disturb the bead pellet.

12. An aliquot of quantitated single-stranded template DNA (sstDNA) wasremoved. The final concentration was 200,000-sst DNA molecules/μl.

13. 3 μl of the diluted sstDNA was added to PCR tube containing thebeads. This was equivalent to 600,000 copies of sstDNA.

14. The tube was vortexed gently to mix contents.

15. The sstDNA was annealed to the capture beads in a PCR thermocyclerwith the program 80 Anneal stored in the EPCR folder on the M JThermocycler, using the following protocol:

-   -   5 minutes at 65° C.;    -   Decrease by 0.1° C./sec to 60° C.;    -   Hold at 60° C. for 1 minute;    -   Decrease by 0.1° C./sec to 50° C.;    -   Hold at 50° C. for 1 minute;    -   Decrease by 0.1° C./sec to 40° C.;    -   Hold at 40° C. for 1 minute;    -   Decrease by 0.1° C./sec to 20° C.; and    -   Hold at 10° C. until ready for next step.

In most cases, beads were used for amplification immediately aftertemplate binding. If beads were not used immediately, they should werestored in the template solution at 4° C. until needed. After storage,the beads were treated as follows.

16. As in step 6, the beads were removed from the thermocycler,centrifuged, and annealing buffer was removed without disturbing thebeads.

17. The beads were stored in an ice bucket until emulsification (Example2).

18. The capture beads included, on average, 0.5 to 1 copies of sstDNAbound to each bead, and were ready for emulsification.

Example 2 Emulsification

This example describes how to create a heat-stable water-in-oil emulsioncontaining about 3,000 PCR microreactors per microliter. Outlined belowis a protocol for preparing the emulsion.

1. 200 μl of PCR solution was added to the 600,000 beads (bothcomponents from Example 1).

2. The solution was pipetted up and down several times to resuspend thebeads.

3. The PCR-bead mixture was allowed to incubate at room temperature for2 minutes to equilibrate the beads with PCR solution.

4. 400 μl of Emulsion Oil was added to a UV-irradiated 2 ml microfugetube.

5. An “amplicon-free” ¼″ stir magnetic stir bar was added to the tube ofEmulsion Oil.

An amplicon-free stir bar was prepared as follows. A large stir bar wasused to hold a ¼″ stir bar. The stir bar was then:

-   -   Washed with DNA-Off (drip or spray);    -   Rinsed with picopure water;    -   Dried with a Kimwipe edge; and    -   UV irradiated for 5 minutes.

6. The magnetic insert of a Dynal MPC-S tube holder was removed. Thetube of Emulsion Oil was placed in the tube holder. The tube was set inthe center of a stir plate set at 600 rpm.

7. The tube was vortexed extensively to resuspend the beads. Thisensured that there was minimal clumping of beads.

8. Using a P-200 pipette, the PCR-bead mixture was added drop-wise tothe spinning oil at a rate of about one drop every 2 seconds, allowingeach drop to sink to the level of the magnetic stir bar and becomeemulsified before adding the next drop. The solution turned into ahomogeneous milky white liquid with a viscosity similar to mayonnaise.

9. Once the entire PCR-bead mixture was been added, the microfuge tubewas flicked a few times to mix any oil at the surface with the milkyemulsion.

10. Stirring was continued for another 5 minutes.

11. Steps 9 and 10 were repeated.

12. The stir bar was removed from the emulsified material by dragging itout of the tube with a larger stir bar.

13. 10 μL of the emulsion was removed and placed on a microscope slide.The emulsion was covered with a cover slip and the emulsion wasinspected at 50× magnification (10× ocular and 5× objective lens). A“good” emulsion was expected to include primarily single beads inisolated droplets (microreactors) of PCR solution in oil.

14. A suitable emulsion oil mixture with emulsion stabilizers was madeas follows. The components for the emulsion mixture are shown in Table4. TABLE 4 Quantity Ingredient Required Source Ref. Number Sigma Light94.5 g Sigma M-5904 Mineral Oil Atlox 4912   1 g Uniqema NA Span 80  4.5g Uniqema NA

The emulsion oil mixture was made by prewarming the Atlox 4912 to 60° C.in a water bath. Then, 4.5 grams of Span 80 was added to 94.5 grams ofmineral oil to form a mixture. Then, one gram of the prewarmed Atlox4912 was added to the mixture. The solutions were placed in a closedcontainer and mixed by shaking and inversion. Any sign that the Atloxwas settling or solidifying was remedied by warming the mixture to 60°C., followed by additional shaking.

Example 3 Amplification

This example describes amplification of the template DNA in thebead—emulsion mixture. According to this protocol of the invention, theDNA amplification phase of the processtakes 3 to 4 hours. After theamplification is complete, the emulsion may be left on the thermocyclerfor up to 12 hours before beginning the process of isolating the beads.PCR thermocycling was performed by placing 50 to 100 μl of theemulsified reaction mixture into individual PCR reaction chambers (i.e.,PCR tubes). PCR was performed as follows:

1. The emulsion was transferred in 50-100 μL amounts into approximately10 separate PCR tubes or a 96-well plate using a single pipette tip. Forthis step, the water-in-oil emulsion was highly viscous.

2. The plate was sealed, or the PCR tube lids were closed, and thecontainers were placed into in a MJ thermocycler with or without a96-well plate adaptor.

3. The PCR thermocycler was programmed to run the following program:

-   -   1 cycle (4 minutes at 94° C.)—Hotstart Initiation;    -   40 cycles (30 seconds at 94° C., 30 seconds at 58° C., 90        seconds at 68° C.);    -   25 cycles (30 seconds at 94° C., 6 minutes at 58° C.); and    -   Storage at 14° C.

4. After completion of the PCR reaction, the amplified material wasremoved in order to proceed with breaking the emulsion and beadrecovery.

Example 4 Breaking the Emulsion and Bead Recovery

This example describes how to break the emulsion and recover the beadswith amplified template thereon. Preferably, the post-PCR emulsionshould remain intact. The lower phase of the emulsion should, by visualinspection, remain a milky white suspension. If the solution is clear,the emulsion may have partially resolved into its aqueous and oilphases, and it is likely that many of the beads will have a mixture oftemplates. If the emulsion has broken in one or two of the tubes, thesesamples should not be combined with the others. If the emulsion hasbroken in all of the tubes, the procedure should not be continued.

1. All PCR reactions from the original 600 μl sample were combined intoa single 1.5 ml microfuge tube using a single pipette tip. As indicatedabove, the emulsion was quite viscous. In some cases, pipetting wasrepeated several times for each tube. As much material as possible wastransferred to the 1.5 ml tube.

2. The remaining emulsified material was recovered from each PCR tube byadding 50 μl of Sigma Mineral Oil into each sample. Using a singlepipette tip, each tube was pipetted up and down a few times to resuspendthe remaining material.

3. This material was added to the 1.5 ml tube containing the bulk of theemulsified material.

4. The sample was vortexed for 30 seconds.

5. The sample was spun for 20 minutes in the tabletop microfuge tube at13.2K rpm in the Eppendorf microcentrifuge.

6. The emulsion separated into two phases with a large white interface.As much of the top, clear oil phase as possible was removed. The cloudymaterial was left in the tube. Often a white layer separated the oil andaqueous layers. Beads were often observed pelleted at the bottom of thetube.

7. The aqueous layer above the beads was removed and saved for analysis(gel analysis, Agilent 2100, and Taqman). If an interface of whitematerial persisted above the aqueous layer, 20 microliters of theunderlying aqueous layer was removed. This was performed by penetratingthe interface material with a pipette tip and withdrawing the solutionfrom underneath.

8. In the PTP Fabrication and Surface Chemistry Room Fume Hood, 1 ml ofHexanes was added to the remainder of the emulsion.

9. The sample was vortexed for 1 minute and spun at full speed for 1minute.

10. In the PTP Fabrication and Surface Chemistry Room Fume Hood, thetop, oil/hexane phase was removed and placed into the organic wastecontainer.

11. 1 ml of 1× Annealing Buffer was added in 80% Ethanol to theremaining aqueous phase, interface, and beads.

12. The sample was vortexed for 1 minute or until the white substancedissolved.

13. The sample was centrifuged for 1 minute at high speed. The tube wasrotated 180 degrees, and spun again for 1 minute. The supernatant wasremoved without disturbing the bead pellet.

14. The beads were washed with 1 ml of 1× Annealing Buffer containing0.1% Tween 20 and this step was repeated.

Example 5 Single Strand Removal and Primer Annealing

If the beads are to be used in a pyrophosphate-based sequencingreaction, then it is necessary to remove the second strand of the PCRproduct and anneal a sequencing primer to the single stranded templatethat is bound to the bead. This example describes a protocol foraccomplishing that.

1. The beads were washed with 1 ml of water, and spun twice for 1minute. The tube was rotated 180° between spins. After spinning, theaqueous phase was removed.

2. The beads were washed with 1 ml of 1 mM EDTA. The tube was spun as instep 1 and the aqueous phase was removed.

3. 1 ml of 0.125 M NaOH was added and the sample was incubated for 8minutes.

4. The sample was vortexed briefly and placed in a microcentrifuge.

5. After 6 minutes, the beads were pelleted as in step 1 and as muchsolution as possible was removed.

6. At the completion of the 8 minute NaOH incubation, 1 ml of 1×Annealing Buffer was added.

7. The sample was briefly vortexed, and the beads were pelleted as instep 1. As much supernatant as possible was removed, and another 1 ml of1× Annealing buffer was added.

8. The sample was briefly vortexed, the beads were pelleted as in step1, and 800 μl of 1× Annealing Buffer was removed.

9. The beads were transferred to a 0.2 ml PCR tube.

10. The beads were transferred and as much Annealing Buffer as possiblewas removed, without disturbing the beads.

11. 100 μl of 1× Annealing Buffer was added.

12. 4 μl of 100 μM sequencing primer was added. The sample was vortexedjust prior to annealing.

13. Annealing was performed in a MJ thermocycler using the “80 Anneal”program.

14. The beads were washed three times with 200 μl of 1× Annealing Bufferand resuspended with 100 μl of 1× Annealing Buffer.

15. The beads were counted in a Hausser Hemacytometer. Typically,300,000 to 500,000 beads were recovered (3,000-5,000 beads/μL).

16. Beads were stored at 4° C. and could be used for sequencing for 1week.

Example 6 Optional Enrichment Step

The beads may be enriched for amplicon containing bead using thefollowing procedure. Enrichment is not necessary but it could be used tomake subsequent molecular biology techniques, such as DNA sequencing,more efficient.

Fifty microliters of 10 μM (total 500 pmoles) of biotin-sequencingprimer was added to the Sepharose beads containing amplicons fromExample 5. The beads were placed in a thermocycler. The primer wasannealed to the DNA on the bead by the thermocycler annealing program ofExample 2.

After annealing, the sepharose beads were washed three times withAnnealing Buffer containing 0.1% Tween 20. The beads, now containingssDNA fragments annealed with biotin-sequencing primers, wereconcentrated by centrifugation and resuspended in 200 μl of BST bindingbuffer. Ten microliters of 50,000 unit/ml Bst-polymerase was added tothe resuspended beads and the vessel holding the beads was placed on arotator for five minutes. Two microliters of 10 mM dNTP mixture (i.e.,2.5 μl each of 10 mM dATP, dGTP, dCTP and dTTP) was added and themixture was incubated for an additional 10 minutes at room temperature.The beads were washed three times with annealing buffer containing 0.1 %Tween 20 and resuspended in the original volume of annealing buffer.

Fifty microliters of Dynal Streptavidin beads (Dynal Biotech Inc., LakeSuccess, NY; M270 or MyOne™ beads at 10 mg/ml) was washed three timeswith Annealing Buffer containing 0.1% Tween 20 and resuspended in theoriginal volume in Annealing Buffer containing 0.1% Tween 20. Then theDynal bead mixture was added to the resuspended sepharose beads. Themixture was vortexed and placed in a rotator for 10 minutes at roomtemperature.

The beads were collected on the bottom of the test tube bycentrifugation at 2300 g (500 rpm for Eppendorf Centrifuge 5415D). Thebeads were resuspended in the original volume of Annealing Buffercontaining 0.1% Tween 20. The mixture, in a test tube, was placed in amagnetic separator (Dynal). The beads were washed three times withAnnealing Buffer containing 0.1% Tween 20 and resuspended in theoriginal volume in the same buffer. The beads without amplicons wereremoved by wash steps, as previously described. Only Sepharose beadscontaining the appropriated DNA fragments were retained.

The magnetic beads were separated from the sepharose beads by additionof 500 μl of 0.125 M NaOH. The mixture was vortexed and the magneticbeads were removed by magnetic separation. The Sepharose beads remainingin solution was transferred to another tube and washed with 400 μl of 50mM Tris Acetate until the pH was stabilized at 7.6.

Example 7 Nucleic Acid Sequencing Using Bead Emulsion PCR

The following experiment was performed to test the efficacy of the beademulsion PCR. For this protocol, 600,000 Sepharose beads, with anaverage diameter of 25-35 μm (as supplied my the manufacturer) werecovalently attached to capture primers at a ratio of 30-50 millioncopies per bead. The beads with covalently attached capture primers weremixed with 1.2 million copies of single stranded Adenovirus Library. Thelibrary constructs included a sequence that was complimentary to thecapture primer on the beads.

The adenovirus library was annealed to the beads using the proceduredescribed in Example 1. Then, the beads were resuspended in complete PCRsolution. The PCR Solution and beads were emulsified in 2 volumes ofspinning emulsification oil using the same procedure described inExample 2. The emulsified (encapsulated) beads were subjected toamplification by PCR as outlined in Example 3. The emulsion was brokenas outlined in Example 4. DNA on beads was rendered single stranded,sequencing primer was annealed using the procedure of Example 5.

Next, 70,000 beads were sequenced simultaneously by pyrophosphatesequencing using a pyrophosphate sequencer from 454 Life Sciences (NewHaven, Conn.) (see copending application of Lohman et al., filedconcurrently herewith entitled Methods of Amplifying and SequencingNucleic Acids” U.S. Ser. No. 60/476,592 filed Jun. 6, 2003). Multiplebatches of 70,000 beads were sequenced and the data were listed in Table5, below. TABLE 5 Alignment Error Alignments Tolerance None SingleMultiple Unique Coverage Inferred Read Error  0% 47916 1560 1110 54.98%0.00%  5% 46026 3450 2357 83.16% 1.88% 10% 43474 6001 1 3742 95.64%4.36%

Table 5 shows the results obtained from BLAST analysis comparing thesequences obtained from the pyrophosphate sequencer against Adenovirussequence. The first column shows the error tolerance used in the BLASTprogram. The last column shows the real error as determined by directcomparison to the known sequence.

Bead Emulsion PCR for Doubled Ended Sequencing

Example 8 Template Quality Control

As indicated previously, the success of the Emulsion PCR reaction wasfound to be related to the quality of the single stranded templatespecies. Accordingly, the quality of the template material was assessedwith two separate quality controls before initiating the Emulsion PCRprotocol. First, an aliquot of the single-stranded template was run onthe 2100 BioAnalyzer (Agilient). An RNA Pico Chip was used to verifythat the sample included a heterogeneous population of fragments,ranging in size from approximately 200 to 500 bases. Second, the librarywas quantitated using the RiboGreen fluorescence assay on a Bio-TekFL600 plate fluorometer. Samples determined to have DNA concentrationsbelow 5 ng/μl were deemed too dilute for use.

Example 9 DNA Capture Bead Synthesis

Packed beads from a 1 mL N-hydroxysuccinimide ester (NHS)-activatedSepharose HP affinity column (Amersham Biosciences, Piscataway, N.J.)were removed from the column. The 30 -25 μm size beads were selected byserial passage through 30 and 25 μm pore filter mesh sections (SefarAmerica, Depew, N.Y., USA). Beads that passed through the first filter,but were retained by the second were collected and activated asdescribed in the product literature (Amersham Pharmacia Protocol #71700600AP). Two different amine-labeled HEG (hexaethyleneglycol) longcapture primers were obtained, corresponding to the 5′ end of the senseand antisense strand of the template to be amplified, (5′-Amine-3 HEGspacers gcttacctgaccgacctctgcctatcccctgttgcgtgtc-3′; SEQ ID NO:1; and5′-Amine-3 HEG spacers ccattccccagctcgtcttgccatctgttccctccctgtc-3′; SEQID NO:2) (IDT Technologies, Coralville, Iowa, USA). The primers weredesigned to capture of both strands of the amplification products toallow double ended sequencing, i.e., sequencing the first and secondstrands of the amplification products. The capture primers weredissolved in 20 mM phosphate buffer, pH 8.0, to obtain a finalconcentration of 1 mM. Three microliters of each primer were bound tothe sieved 30-25 μm beads. The beads were then stored in a bead storagebuffer (50 mM Tris, 0.02% Tween and 0.02% sodium azide, pH 8). The beadswere quantitated with a hemacytometer (Hausser Scientific, Horsham, Pa.,USA) and stored at 4° C. until needed.

Example 10 PCR Reaction Mix Preparation and Formulation

As with any single molecule amplification technique, contamination ofthe reactions with foreign or residual amplicon from other experimentscould interfere with a sequencing run. To reduce the possibility ofcontamination, the PCR reaction mix was prepared in a in a UV-treatedlaminar flow hood located in a PCR clean room. For each 600,000 beademulsion PCR reaction, the following reagents were mixed in a 1.5 mltube: 225 μl of reaction mixture (1× Platinum HiFi Buffer (Invitrogen)),1 mM dNTPs, 2.5 mM MgSO₄ (Invitrogen), 0.1% BSA, 0.01% Tween, 0.003 U/μlthermostable PPi-ase (NEB), 0.125 μM forward primer(5′-gcttacctgaccgacctctg-3′; SEQ ID NO:3) and 0.125 μM reverse primer(5′-ccattccccagctcgtcttg-3′; SEQ ID NO:4) (IDT Technologies, Coralville,Iowa, USA) and 0.2 U/μl Platinum Hi-Fi Taq Polymerase (Invitrogen).Twenty-five microliters of the reaction mixture was removed and storedin an individual 200 μl PCR tube for use as a negative control. Both thereaction mixture and negative controls were stored on ice until needed.

Example 11 Binding Template Species to DNA Capture Beads

Successful clonal DNA amplification for sequencing relates to thedelivery of a controlled number of template species to each bead. Forthe experiments described herein below, the typical target templateconcentration was determined to be 0.5 template copies per capture bead.At this concentration, Poisson distribution dictates that 61% of thebeads have no associated template, 30% have one species of template, and9% have two or more template species. Delivery of excess species canresult in the binding and subsequent amplification of a mixed population(2 or more species) on a single bead, preventing the generation ofmeaningful sequence data. However, delivery of too few species willresult in fewer wells containing template (one species per bead),reducing the extent of sequencing coverage. Consequently, it was deemedthat the single-stranded library template concentration was important.

Template nucleic acid molecules were annealed to complimentary primerson the DNA capture beads by the following method, conducted in aUV-treated laminar flow hood. Six hundred thousand DNA capture beadssuspended in bead storage buffer (see Example 9, above) were transferredto a 200 μl PCR tube. The tube was centrifuged in a benchtop minicentrifuge for 10 seconds, rotated 180°, and spun for an additional 10seconds to ensure even pellet formation. The supernatant was removed,and the beads were washed with 200 μl of Annealing Buffer (20 mM Tris,pH 7.5 and 5 mM magnesium acetate). The tube was vortexed for 5 secondsto resuspend the beads, and the beads were pelleted as before. All butapproximately 10 μl of the supernatant above the beads was removed, andan additional 200 μl of Annealing Buffer was added. The beads were againvortexed for 5 seconds, allowed to sit for 1 minute, and then pelletedas before. All but 10 μl of supernatant was discarded.

Next, 1.5 μl of 300,000 molecules/μl template library was added to thebeads. The tube was vortexed for 5 seconds to mix the contents, and thetemplates were annealed to the beads in a controlleddenaturation/annealing program preformed in an MJ thermocycler. Theprogram allowed incubation for 5 minutes at 80° C., followed by adecrease by 0. 1° C./sec to 70° C., incubation for 1 minute at 70° C.,decrease by 0.1° C./s 60° C., hold at 60° C. for 1 minute, decrease by0.1° C./sec to 50° C., hold at 50° C. for 1 minute, decrease by 0.1°C./sec to 20° C., hold at 20° C. Following completion of the annealingprocess, the beads were removed from the thermocycler, centrifuged asbefore, and the Annealing Buffer was carefully decanted. The capturebeads included on average 0.5 copy of single stranded template DNA boundto each bead, and were stored on ice until needed.

Example 12 Emulsification

The emulsification process creates a heat-stable water-in-oil emulsioncontaining 10,000 discrete PCR microreactors per microliter. This servesas a matrix for single molecule, clonal amplification of the individualmolecules of the target library. The reaction mixture and DNA capturebeads for a single reaction were emulsified in the following manner. Ina UV-treated laminar flow hood, 200 μl of PCR solution (from Example 10)was added to the tube containing the 600,000 DNA capture beads (fromExample 11). The beads were resuspended through repeated pipetting.After this, the PCR-bead mixture was incubated at room temperature forat least 2 minutes, allowing the beads to equilibrate with the PCRsolution. At the same time, 450 μl of Emulsion Oil (4.5 % (w:w) Span 80,1% (w:w) Atlox 4912 (Uniqema, Del.) in light mineral oil (Sigma)) wasaliquotted into a flat-topped 2 ml centrifuge tube (Dot Scientific)containing a sterile ¼ inch magnetic stir bar (Fischer). This tube wasthen placed in a custom-made plastic tube holding jig, which was thencentered on a Fisher Isotemp digital stirring hotplate (FisherScientific) set to 450 RPM.

The PCR-bead solution was vortexed for 15 seconds to resuspend thebeads. The solution was then drawn into a 1 ml disposable plasticsyringe (Benton-Dickenson) affixed with a plastic safety syringe needle(Henry Schein). The syringe was placed into a syringe pump (Cole-Parmer)modified with an aluminum base unit orienting the pump vertically ratherthan horizontally (e.g., FIGS. 4-6). The tube with the emulsion oil wasaligned on the stir plate so that it was centered below the plasticsyringe needle and the magnetic stir bar was spinning properly. Thesyringe pump was set to dispense 0.6 ml at 5.5 ml/hr. The PCR-beadsolution was added to the emulsion oil in a dropwise fashion. Care wastaken to ensure that the droplets did not contact the side of the tubeas they fell into the spinning oil.

Once the emulsion was formed, great care was taken to minimize agitationof the emulsion during both the emulsification process and thepost-emulsification aliquotting steps. It was found that vortexing,rapid pipetting, or excessive mixing could cause the emulsion to break,destroying the discrete microreactors. In forming the emulsion, the twosolutions turned into a homogeneous milky white mixture with theviscosity of mayonnaise. The contents of the syringe were emptied intothe spinning oil. Then, the emulsion tube was removed from the holdingjig, and gently flicked with a forefinger until any residual oil layerat the top of the emulsion disappeared. The tube was replaced in theholding jig, and stirred with the magnetic stir bar for an additionalminute. The stir bar was removed from the emulsion by running a magneticretrieval tool along the outside of the tube, and the stir bar wasdiscarded.

Twenty microliters of the emulsion was taken from the middle of the tubeusing a P100 pipettor and placed on a microscope slide. The largerpipette tips were used to minimize shear forces. The emulsion wasinspected at 50× magnification to ensure that it was comprisedpredominantly of single beads in 30 to 150 micron diameter microreactorsof PCR solution in oil (FIG. 7). After visual examination, the emulsionswere immediately amplified.

Example 13 Amplification

The emulsion was aliquotted into 7-8 separate PCR tubes. Each tubeincluded approximately 75 μl of the emulsion. The tubes were sealed andplaced in a MJ thermocycler along with the 25 μl negative controldescribed above. The following cycle times were used: 1 cycle ofincubation for 4 minutes at 94° C. (Hotstart Initiation), 30 cycles ofincubation for 30 seconds at 94° C., and 150 seconds at 68° C.(Amplification), and 40 cycles of incubation for 30 seconds at 94° C.,and 360 seconds at 68° C. (Hybridization and Extension). Aftercompletion of the PCR program, the tubes were removed and the emulsionswere broken immediately or the reactions were stored at 10° C. for up to16 hours prior to initiating the breaking process.

Example 14 Breaking the Emulsion and Bead Recovery

Following amplification, the emulstifications were examined for breakage(separation of the oil and water phases). Unbroken emulsions werecombined into a single 1.5 ml microcentrifuge tube, while the occasionalbroken emulsion was discarded. As the emulsion samples were quiteviscous, significant amounts remained in each PCR tube. The emulsionremaining in the tubes was recovered by adding 75 μl of mineral oil intoeach PCR tube and pipetting the mixture. This mixture was added to the1.5 ml tube containing the bulk of the emulsified material. The 1.5 mltube was then vortexed for 30 seconds. After this, the tube wascentrifuged for 20 minutes in the benchtop microcentrifuge at 13.2K rpm(full speed).

After centrifugation, the emulsion separated into two phases with alarge white interface. The clear, upper oil phase was discarded, whilethe cloudy interface material was left in the tube. In a chemical fumehood, 1 ml hexanes was added to the lower phase and interface layer. Themixture was vortexed for 1 minute and centrifuged at full speed for Iminute in a benchtop microcentrifuge. The top, oil/hexane phase wasremoved and discarded. After this, 1 ml of 80% Ethanol/1× AnnealingBuffer was added to the remaining aqueous phase, interface, and beads.This mixture was vortexed for 1 minute or until the white material fromthe interface was dissolved. The sample was then centrifuged in abenchtop microcentrifuge for 1 minute at full speed. The tube wasrotated 180 degrees, and spun again for an additional minute. Thesupernatant was then carefully removed without disturbing the beadpellet.

The white bead pellet was washed twice with 1 ml Annealing Buffercontaining 0.1% Tween 20. The wash solution was discarded and the beadswere pelleted after each wash as described above. The pellet was washedwith 1 ml Picopure water. The beads were pelleted with thecentrifuge-rotate-centrifuge method used previously. The aqueous phasewas carefully removed. The beads were then washed with 1 ml of 1 mM EDTAas before, except that the beads were briefly vortexed at a mediumsetting for 2 seconds prior to pelleting and supernatant removal.

Amplified DNA, immobilized on the capture beads, was treated to obtainsingle stranded DNA. The second strand was removed by incubation in abasic melt solution. One ml of Melt Solution (0.125 M NaOH, 0.2 M NaCl)was subsequently added to the beads. The pellet was resuspended byvortexing at a medium setting for 2 seconds, and the tube placed in aThermolyne LabQuake tube roller for 3 minutes. The beads were thenpelleted as above, and the supernatant was carefully removed anddiscarded. The residual Melt solution was neutralized by the addition of1 ml Annealing Buffer. After this, the beads were vortexed at mediumspeed for 2 seconds. The beads were pelleted, and the supernatant wasremoved as before. The Annealing Buffer wash was repeated, except thatonly 800 μl of the Annealing Buffer was removed after centrifugation.The beads and remaining Annealing Buffer were transferred to a 0.2 mlPCR tube. The beads were used immediately or stored at 4° C. for up to48 hours before continuing on to the enrichment process.

Example 15 Bead Enrichment

The bead mass included beads with amplified, immobilized DNA strands,and empty or null beads. As mentioned previously, it was calculated that61% of the beads lacked template DNA during the amplification process.Enrichment was used to selectively isolate beads with template DNA,thereby maximizing sequencing efficiency. The enrichment process isdescribed in detail below.

The single stranded beads from Example 14 were pelleted with thecentrifuge-rotate-centrifuge method, and as much supernatant as possiblewas removed without disturbing the beads. Fifteen microliters ofAnnealing Buffer were added to the beads, followed by 2 μl of 100 μMbiotinylated, 40 base enrichment primer (5′-Biotin-tetra-ethyleneglycolspacers ccattccccagctcgtcttgccatctgttccctccctgtctcag-3′; SEQ ID NO:5).The primer was complimentary to the combined amplification andsequencing sites (each 20 bases in length) on the 3′ end of thebead-immobilized template. The solution was mixed by vortexing at amedium setting for 2 seconds, and the enrichment primers were annealedto the immobilized DNA strands using a controlled denaturation/annealingprogram in an MJ thermocycler. The program consisted of the followingcycle times and temperatures: incubation for 30 seconds at 65° C.,decrease by 0.1° C./sec to 58° C., incubation for 90 seconds at 58° C.,and hold at 10° C.

While the primers were annealing, Dynal MyOne™ streptavidin beads wereresuspend by gentle swirling. Next, 20 μl of the MyOne™ beads were addedto a 1.5 ml microcentrifuge tube containing 1 ml of Enhancing fluid (2 MNaCl, 10 mM Tris-HCl, 1 mM EDTA, pH 7.5). The MyOne bead mixture wasvortexed for 5 seconds, and the tube was placed in a Dynal MPC-S magnet.The paramagnetic beads were pelleted against the side of themicrocentrifuge tube. The supernatant was carefully removed anddiscarded without disturbing the MyOne™ beads. The tube was removed fromthe magnet, and 100 μl of enhancing fluid was added. The tube wasvortexed for 3 seconds to resuspend the beads, and stored on ice untilneeded.

Upon completion of the annealing program, 100 μl of annealing buffer wasadded to the PCR tube containing the DNA capture beads and enrichmentprimer. The tube vortexed for 5 seconds, and the contents weretransferred to a fresh 1.5 ml microcentrifuge tube. The PCR tube inwhich the enrichment primer was annealed to the capture beads was washedonce with 200 μl of annealing buffer, and the wash solution was added tothe 1.5 ml tube. The beads were washed three times with 1 ml ofannealing buffer, vortexed for 2 seconds, and pelleted as before. Thesupernatant was carefully removed. After the third wash, the beads werewashed twice with 1 ml of ice cold Enhancing fluid. The beads werevortexed, pelleted, and the supernatant was removed as before. The beadswere resuspended in 150 μl ice cold Enhancing fluid and the beadsolution was added to the washed MyOne™ beads.

The bead mixture was vortexed for 3 seconds and incubated at roomtemperature for 3 minutes on a LabQuake tube roller. Thestreptavidin-coated MyOne™ beads were bound to the biotinylatedenrichment primers annealed to immobilized templates on the DNA capturebeads. The beads were then centrifuged at 2,000 RPM for 3 minutes, afterwhich the beads were vortexed with 2 second pulses until resuspended.The resuspended beads were placed on ice for 5 minutes. Following this,500 μl of cold Enhancing fluid was added to the beads and the tube wasinserted into a Dynal MPC-S magnet. The beads were left undisturbed for60 seconds to allow pelleting against the magnet. After this, thesupernatant with excess MyOne™ and null DNA capture beads was carefullyremoved and discarded.

The tube was removed from the MPC-S magnet, and 1 ml of cold enhancingfluid added to the beads. The beads were resuspended with gentle fingerflicking. It was important not to vortex the beads at this time, asforceful mixing could break the link between the MyOne™ and DNA capturebeads. The beads were returned to the magnet, and the supernatantremoved. This wash was repeated three additional times to ensure removalof all null capture beads. To remove the annealed enrichment primers andMyOne™ beads, the DNA capture beads were resuspended in 400 μl ofmelting solution, vortexed for 5 seconds, and pelleted with the magnet.The supernatant with the enriched beads was transferred to a separate1.5 ml microcentrifuge tube. For maximum recovery of the enriched beads,a second 400 μl aliquot of melting solution was added to the tubecontaining the MyOne™ beads. The beads were vortexed and pelleted asbefore. The supernatant from the second wash was removed and combinedwith the first bolus of enriched beads. The tube of spent MyOne™ beadswas discarded.

The microcentrifuge tube of enriched DNA capture beads was placed on theDynal MPC-S magnet to pellet any residual MyOne™ beads. The enrichedbeads in the supernatant were transferred to a second 1.5 mlmicrocentrifuge tube and centrifuged. The supernatant was removed, andthe beads were washed 3 times with 1 ml of annealing buffer toneutralize the residual melting solution. After the third wash, 800 μlof the supernatant was removed, and the remaining beads and solutionwere transferred to a 0.2 ml PCR tube. The enriched beads werecentrifuged at 2,000 RPM for 3 minutes and the supernatant decanted.Next, 20 μl of annealing buffer and 3 μl of two different 100 μMsequencing primers (5′-ccatctgttccctccctgtc-3′; SEQ ID NO:6; and5′-cctatcccctgttgcgtgtc-3′ phosphate; SEQ ID NO:7) were added. The tubewas vortexed for 5 seconds, and placed in an MJ thermocycler for thefollowing 4-stage annealing program: incubation for 5 minutes at 65° C.,decrease by 0.1° C./sec to 50° C., incubation for 1 minute at 50° C.,decrease by 0. 1° C./sec to 40° C., hold at 40° C. for 1 minute, de 0.1°C./sec to 15° C., and hold at 15° C.

Upon completion of the annealing program, the beads were removed fromthermocycler and pelleted by centrifugation for 10 seconds. The tube wasrotated 180°, and spun for an additional 10 seconds. The supernatant wasdecanted and discarded, and 200 μl of annealing buffer was added to thetube. The beads were resuspended with a 5 second vortex, and pelleted asbefore. The supernatant was removed, and the beads resuspended in 100 μlannealing buffer. At this point, the beads were quantitated with aMultisizer 3 Coulter Counter (Beckman Coulter). Beads were stored at 4°C. and were stable for at least 1 week.

Throughout this specification, various patents, published patentapplications and scientific references are cited to describe the stateand content of the art. Those disclosures, in their entireties, arehereby incorporated into the present specification by reference.

Although particular embodiments have been disclosed herein in detail,this has been done by way of example for purposes of illustration only,and is not intended to be limiting with respect to the scope of theappended claims, which follow. In particular, it is contemplated by theinventors that various substitutions, alterations, and modifications maybe made to the invention without departing from the spirit and scope ofthe invention as defined by the claims.

1. A method for amplifying one or more nucleic acids comprising thesteps of: (a) forming a water-in-oil emulsion to create a plurality ofaqueous microreactors wherein at least one of the microreactorscomprises a single nucleic acid template, a single bead capable ofbinding to the nucleic acid, and amplification reaction solutioncontaining reagents necessary to perform nucleic acid amplification; (b)amplifying the nucleic acids in the microreactors to form amplifiedcopies of said nucleic acids; and (c) binding the amplified copies tothe beads in the microreactors.
 2. The method of claim 1, wherein amajority of the microreactors include a single nucleic acid.
 3. Themethod of claim 1, wherein said amplification reaction solution is apolymerase chain reaction solution comprising nucleotide triphosphates,a thermostable polymerase, and nucleic acid primers suspended in abuffer compatible with polymerase chain reaction conditions.
 4. Themethod of claim 3, wherein said polymerase chain reaction is anasymmetric polymerase chain reaction.
 5. The method of claim 3, whereinsaid polymerase chain reaction is a symmetric polymerase chain reaction.6. The method of claim 1, wherein said emulsion additionally containsemulsion stabilizers.
 7. The method of claim 6, wherein said emulsionstabilizers are selected from the group consisting of Atlox 4912, Span80, and combinations and mixtures thereof.
 8. The method of claim 1wherein said emulsion is heat stable.
 9. The method of claim 8 whereinsaid emulsion is heat stable to 95° C.
 10. The method of claim 1,wherein amplification is carried out by a method selected from the groupconsisting of transcription-based amplification, rapid amplification ofcDNA ends, continuous flow amplification, and rolling circleamplification.
 11. The method of claim 1, wherein the emulsion is formedby the dropwise addition of the nucleic acid templates, beads, andamplification reaction solution into an oil.
 12. The method of claim 1,performed with at least 10,000 nucleic acids.
 13. The method of claim 1,performed with at least 50,000 nucleic acids.
 14. The method of claim 1,wherein the microreactors have an average size of 50 to 250 μm indiameter.
 15. The method of claim 1, wherein after step (c) each beadcaptures more than 10,000 amplification copies of a nucleic acidtemplate.
 16. A library comprising a plurality of nucleic acidmolecules, wherein each nucleic acid molecule is separately immobilizedto a different bead, and wherein each bead comprises over 1,000,000clonal amplification copies of each nucleic acid molecule, wherein thelibrary is contained in a single vessel.
 17. The library of claim 16,wherein the nucleic acid molecules are selected from the groupconsisting of genomic DNA, cDNA, episomal DNA, BAC DNA, and YAC DNA. 18.The library of claim 16, wherein the genomic DNA is selected from thegroup consisting of animal, plant, viral, bacterial, and fungal genomicDNA.
 19. The library of claim 18, wherein the genomic DNA is humangenomic DNA or human cDNA.
 20. The library of claim 16, wherein the beadhas a diameter of 2 microns to 100 microns.
 21. The library of claim 16,wherein the bead is a sepharose bead.
 22. A method for amplifying anucleic acid comprising the steps of: (a) providing a nucleic acidtemplate to be amplified; (b) providing a solid support materialcomprising a generally spherical bead having a diameter about 10 toabout 80 μm, wherein the bead is capable of binding to the nucleic acidtemplate; (c) mixing the nucleic acid template and the bead in anamplification reaction solution containing reagents necessary to performa nucleic acid amplification reaction in a water-in-oil emulsion; (d)amplifying the nucleic acid template to form amplified copies of saidnucleic acid template; and (e) binding the amplified copies to the bead.23. A kit for conducting nucleic acid amplification of a nucleic acidtemplate comprising: (a) a nucleic acid capture bead; (b) an emulsionoil; (c) one or more emulsion stabilizers; (d) instructions forperforming the method of claim 1 or claim
 22. 24. The method of claim 1or claim 22 further comprising the step of enriching for beads whichbind amplified copies of the nucleic acid away from beads to which nonucleic acid is bound, the enrichment step selected from the groupconsisting of affinity purification, electrophoresis and cell sorting.25. The method of claim 24 wherein the enrichment step is performed byaffinity purification with magnetic beads that bind nucleic acid. 26.The method of claim 1 or 22, wherein at least 100,000 copies of eachtarget nucleic acid molecule are bound to each bead.
 27. The method ofclaim 1 or 22, wherein at least 1,000,000 copies of each target nucleicacid molecule are bound to each bead.
 28. The method of claim 1 or 22,wherein between at least 1 to 20,000,000 copies of each target nucleicacid molecule are bound to each bead.
 29. The method of claim 1 or 22,wherein the beads are sepharose beads.
 30. The method of claim 1 or 22,wherein amplified copies are bound to the beads by a binding pairselected from the group consisting of antigen/antibody, ligand/receptorand polyhistidine/nickel.
 31. The method of claim 30, wherein thebinding pair is avidin/biotin.
 32. The method of claim 25, furthercomprising the steps of: separating the template carrying beads andmagnetic bead; and removing the magnetic beads with a magnetic field.33. The method of claim 32, wherein the separating is achieved byincubation at a temperature greater than 45° C. or by incubating thetemplate carrying beads and the magnetic beads in a solution with abasic pH.
 34. A method for producing a clonal population of nucleicacids, comprising: (a) providing a plurality of nucleic acid templatesfrom 50-800 bp in length and beads capable of binding to the nucleicacid templates; (b) mixing the nucleic acid templates and the beads in abiological reaction solution containing reagents necessary to amplifythe nucleic acid templates; (c) forming an emulsion to create aplurality of microreactors comprising the nucleic acid templates, beads,and biological reaction solution, wherein at least one of themicroreactors comprises a single nucleic acid template and a single beadencapsulated in the biological reaction solution, wherein themicroreactors are contained in the same vessel.
 35. The method of claim34 further comprising transcribing and translated the nucleic acids togenerate at least 10,000 copies of an expression product.
 36. The methodof claim 35, wherein said expression product is bound to said beads by abinding pair selected from the group consisting of antigen/antibody,ligand/receptor, 6×his/nickel-nitrilotriacetic acid, and FLAG tag/FLAGantibody binding pairs.
 37. The method of claim 35, wherein the methodproduces a clonal population of proteins.
 38. The method of claim 37,wherein the proteins are selected from the group consisting ofantibodies, antibodies fragments, and engineered antibodies.
 39. Anemulsion comprising a plurality of thermostable microreactors, whereinthe microreactors are 50 to 200 μm in diameter and comprise a biologicalreaction solution.
 40. The emulsion of claim 39, wherein the biologicalreaction solution comprises reagents for performing polymerase chainreaction amplification.
 41. The emulsion of claim 39, wherein thebiological reaction solution comprises reagents for performing coupledtranscription and translation reactions.
 42. The emulsion of claim 40 orclaim 41, wherein the plurality of microreactors comprise a nucleic acidtemplate.
 43. The emulsion of claim 42, wherein the plurality ofmicroreactors comprise one or fewer nucleic acid templates.
 44. Theemulsion of claim 43, wherein the plurality of microreactors compriseone or fewer beads that bind to the nucleic acid templates.